Inorg. Chem. 1988,27, 3292-3291
3292
Contribution from the Department of Inorganic Chemistry, Indian Association for the Cultivation of Science, Calcutta 700032, India
Trivalent Nickel. The Quinone Oximate Family: Synthesis and Redox Regulation of Isomerism and Ligand Redistribution Debashis Ray and Animesh Chakravorty* Received February 4, 1988 The anionic N 3 0 3tris chelates Ni"(RQ)d V 0.38 0.39 0.40 0.40 0.6 1 0.63 0.30 0.31 0.37 0.38 0.41 0.40 0.61 0.62 0.29 0.30
E,, V 0.52 0.52 0.55 0.55 0.75 0.77 0.46 0.46 0.51 0.52 0.75 0.44
E,, V 0.46 0.46c 0.49 0.49c 0.69 0.7Ic 0.40 O.4Oe
Eo:d V
n ( E , V)*-'
0.49 0.49 0.52 0.52 0.72 0.74 0.43 0.43
0.98 (0.72) 0.97 (0.72) 1.02 (0.75) 0.93 (0.75) 0.95 (0.97) 0.98 (0.97) 1.00 (0.66) 0.92 (0.66) 0.98 (0.13) 0.96 (0.13) 0.97 (0.15) 1.03 (0.15) 0.99 (0.35) 0.90 (0.35) 0.96 (0.05) 0.91 (0.05)
f f f f
0.488 0.498 0.728 0.419
'Unless otherwise stated, meanings of the symbols are the same as in the text. bThe solvent is acetonitrile, the supporting electrolyte is TEAP (0.1 M), the working electrode is platinum, and the reference electrode is the SCE. cCyclicvoltammetric data: scan rate 50 mV s-I. d E o calculated as the average of anodic and cathodic peak potentials. CShoulderon cyclic voltammogram. /Not observed. SCalculated by assuming that peak-to-peak separation is 60 mV. hConstant-potentialcoulometric data. In = Q/Q', where Q is the observed coulomb count and Q'is the calculated coulomb count for one-electron transfer; E is the constant potential at which electrolysis was performed.
I
02
04 E(V)vsSCE
I
0.2
l
l
04 E ( V ) v s SCE
Figure 2. Variable-temperature cyclic voltammograms (CV) (-) and differential pulse voltammograms (DPV) ( - - - ) of a -IO-, M K[Ni(MeQ),] solution in acetonitrile (0.1 M in TEAP) at a platinum electrode with scan rates of 50 mV s-l (CV) and 10 mV s-' (DPV) and modulation amptitude (DPV) of 25 mV. The marked current range is 20 pA (CV) or 40 pA (DPV).
The R substituent affects the formal potentials in a predictable manner. Thus, the Me substituent lowers the potential significantly more than does the C1 substituent. For the entire group of complexes covering both isomeric and substituent differences the nickel(II1)-nickel(I1) formal potentials cover a substantial range: 0.3-0.7 V (Table 11). Both the mer and the fac couples are nearly reversible, having of 60-70 mV. cyclic voltammetric peak-to-peak separations (Up) Coulometric data (Table 11) demonstrated that the two couples together correspond to the transfer of one electron/mol of Ni(RQ),- (mer + fuc). This as well as forthcoming EPR results militate against the possibility that sequential electron transfer within the same species is the origin of the two couples. In the case of Ni(MeQ),-, the ratio of the anodic peak current of the mer isomer to that of thefuc isomer is 0.75 by cyclic ( 1 7 ) Bond, A. M.; Carr, S. W.; Colton, R. Inorg. Chem. 1984, 23,
2343-2350.
voltammetry and 0.77 by differential pulse voltammetry. These results match very well with the N M R K" value of 0.75. This implies that at 258 K the isomer interconversion rate is slow on the voltammetric time scale as well (scan rate v = 5-50 mV s-I). However, the ratio of the cathodic and anodic peak currents of the fuc isomer is only 0.6-0.7, depending on the switching potential ( u = 10 mV s-l). This is attributed to the homogeneous rate process fuc-Ni(MeQ), -, mer-Ni(MeQ),, which follows the formation of fuc-Ni(MeQ), (3a) by electron transfer at the electrode surface. From current ratios the rate constant ( k ) of the homogeneous process is estimated to be 0.02 sd at 258 K with the help of available procedures.I6 At higher temperatures, determination of k is complicated by the faster rate of isomer interconversion in the bivalent state. The effect of temperature on k can however be qualitatively seen in the voltammograms of Figure 2. The cathodic peak of the fac isomer progressively diminishes with increasing temperature, and at 308 K it is virtually unobservable. Due to the spontaneous nature of fac -, mer conversion, pure Ni(RQ), can be isolated only in the mer form (vide infra). The thermodynamic stability of mer-Ni(MeQ), with respect to the fac isomer has been assessed with the help of the cycle of eq 2 and 7 . If the net free energy change over the cycle is set f r c - NI r1(MeQ)3? - ,
K1', 0 87
1k4QV
mer - N I 'I( M e Q ) 3 39
(2)
v
I11
f8c- NI 'I1( M e Q 13
,
mer - N I 'I*( M e 1 3 ) ~
(
7)
to zero and the stated K" and Eo298values are used, KIrl is estimated to be 43 at 298 K. b. Electrosynthesis and Characterization of mer-Ni(RQ),. Three Ni(RQ), species were isolated in the pure state in 65-70% yield by constant-potential electrolysis of K[Ni(RQ),] in MeCN with NH4PF, as supporting ele~trolyte.'~The initial red color of Ni(RQ),- changed to brown and Ni(RQ), was obtained as a dark powder after removal of solvent and supporting electrolyte. We have not been able to devise a practical chemical oxidation of Ni(RQ),- to Ni(RQ),. Single crystals of Ni(RQ), have eluded us so far. The Ni1"N,06, coordination sphere is known among amino acid and peptide complexes usually in the solution Only one example of a crystalline complex appears (18) Nicholson, R. S.; Shain, I. Anal. Chem. 1964, 36, 706-723. (19) In the case of N ~ ( B Z Qstudies )~ were made on a coulometrically produced solution in dimethylformamide. The insolubility of Na[Ni(BzQ),] in acetonitrile precluded isolation of Ni(BzQ), in pure form.
Inorganic Chemistry, Vol. 27, No. 19, 1988 3295
Trivalent Nickel Table 111. Characterization Data of n~er-Ni(RQ)~ anal.," % b Perf? compd C H N f i ~ g valuesC m e ~ N i ( M e Q ) ~54.19 4.00 8.94 1.95 2.038, 2.138, 2.188 (54.11)
mer-Ni(BuQ)3 60.82 (60.74) me~Ni(C1Q)~ 41.77 (41.67)
(3.86) 6.08 (6.07) 1.87 (1.74)
(9.01) 6.85 (7.09) 8.20 (8.10)
2.03
2.035, 2.138, 2.188
1.90
2.041, 2.138, 2.188 v.100
Calculated values are in parentheses. In the solid state at 298 K. CInacetonitrile-toluene glass at 77 K. a
- V
00
04
E ( V ) v s SCE
08
0.0
0.4
08
E(V)vsSCE
Figure 4. Variable-temperatureand variable-scan-rate(u, mV s-') cyclic voltammograms of lo-' M Ni(MeQ), in acetonitrile (0.1 M in TEAP) at a platinum electrode. The superimposed differential pulse voltammogram ( - - - ) on the right side has u = 10 mV s-I and modulation amptitude 25 mV. The marked current range is 20 PA (CV) or 40 fiA (DPV). Table IV. Mixed Complexes and Their Nickel(II1)-Nickel(I1) Formal Potentials"
H(GI
H(G/
) ~ 1:l Figure 3. EPR spectra (X-band) at 77 K: (a) m e ~ N i ( M e Q in acetonitrile-toluene glass; (b) 30% electrooxidized (263 K) solution of Ni(BuQ),(bpy) in dichloromethaneglass showing the presence of both Ni(B~Q)~(bpy)+ and rner-Ni(B~Q)~; (c) 25% electrooxidized (300 K) solution of Ni(B~Q)~(bpy) in dichloromethane; (d) same as (c) except that the level of electrooxidation is 45%; (e) same as (c) except that the level of electrooxidation is 67%. Identical spectrometer settings were used in spectra c-e. to be known.20b The Ni(RQ), chelates provide new examples. Characterization data for the isolated complexes are set out in Table 111. Magnetic moments correspond to one unpaired electron, and this fits with low-spin nickel(II1) (d'). Pure Ni(RQ), powder generally displays a broad EPR signal near g = 2.12 both at room temperature and at 77 K. In frozen acetonitrile-toluene or dichloromethane glass (77 K), however, the spectra are neatly rhombic (Figure 3a), characterizing the mer geometry of the complex. N o nitrogen hyperfine structure is observed. For the fuc geometry the spectra would have been axial. The two EPR resonances of Ni(RQ), at lower fields are relatively closely spaced and can be considered as components of g, split by a meridional rhombic field. In this idealization the Ni(RQ), complexes belong to the g, > gllclass corresponding to axial elongation (ds ground state).2 The exclusive meridional geometry of isolated Ni(RQ), is similar to the behavior of other M1*'(RQ), chelates (M = Mn,IS Fe,I5 CoZ1). We note that the mer form is sterically favored since here the pendant oximato oxygen atoms have greater separations than in thefac form. This factor is expected to be more important for trivalent than for bivalent ions because of the smaller size of the former. Further, M(RQ), being electrically neutral, thefuc isomer would not be subject to electrostatic stabilization of the type invoked earlier in the case of fuc-M(RQ),-. (20) (a) Lati, J.; Koresh, J.; Meyerstein, D. Chem. Phys. Lett. 1975, 33, 286-288. (b) van der Merwe, M. J.; Boeyens, J. C. A.; Hancock, R. D. Inorg. Chem. 1983,22, 3489-3491. (21) Kalia, K. C.; Kumar, A. Indian J . Chem., Sect. A 1978, 16A, 49-51.
Ni(MeQ),(bpy) 0.71 (90) Ni(MeQ),(m~y)~0.65 (100) Ni(BuQ),(bpy) 0.71 (90) Ni(MeQ),(ap~)~ 0.49 (100) Ni(ClQ),(bpy) 0.88 (90) Ni(MeQ),(pz), 0.80 (120) Ni(MeQ)z(py)z 0.70 (120) "Collected in acetonitrile solutions with use of a platinum working electrode at 298 K, TEAP supporting electrolyte (0.1 M), and SCE reference electrode. bCyclic voltammetric data at a scan rate of 50 mV s-' and solute concentration of M. is calculated as is the peakthe average of anodic and cathodic peak potentials. to-peak separation.
c. Reductive Isomerization. A variable-temperature cyclic voltammogram of Ni(MeQ)3 is displayed in Figure 4. At low temperature a single one-electron couple is observed. The anodic and cathodic peak potentials are identical with those of the mer isomer monitored in the oxidation of Ni(MeQ)< (Table 11). Two conclusions can be drawn: (i) Ni(MeQ)3 exists exclusively in the mer form and (ii) mer-Ni(MeQ),- produced by reduction of ~ z e r - N i ( M e Q )retains ~ stereochemical purity over the cyclic voltammetric time scale at 258 K. When the temperature is raised, electrogenerated mer-Ni(MeQ),- isomerizes faster at the electrode surface and the anodic peak corresponding to thefuc isomer also becomes observable (eq 8). At 278 K isomerization can still be virtually suppressed by n ~ e r - N i ( M e Q )+~ e- + ~ e r - N i ( M e Q ) , -+fac-Ni(MeQ),(8) raising the scan rate to 200 mV s-I. Voltammograms at higher temperature follow the expected pattern. The isomeric purity of mer-Ni(MeQ), is also confirmed by the single differential pulse voltammetric response it displays throughout the studied temperature range (298 K response is shown in Figure 4). The coulomb count in exhaustive reduction of n~er-Ni(MeQ)~ corresponds to the transfer of one electron. Irrespective of the temperature used, an equilibrium mixture offac- and mer-Ni(MeQ)< is produced. Cyclic voltammograms of the reduced solution are identical with those of Figure 2. The electrochemical behavior of other Ni(RQ), complexes is entirely analogous to that of mer-Ni( MeQ)3. C. Mixed Complexes: Oxidative Ligand Redistribution. In the reaction of eq 1, alkaline H R Q can be replaced by amines of the pyridine family such as pyridine (py), 4-methylpyridine (mpy),
3296 Inorganic Chemistry, Vol. 27, No. 19, 1988 4-aminopyridine (apy), pyrazole (pz), and 2,2'-bipyridine (bpy). Selected complexes of the type Ni(RQ),(N,N) formed in this manner are listed in Table IV. The complex Ni(MeQ)z(py), is already known.22 All the complexes of Table IV behave as = 2.79-3.15 pB) and display the two-electron paramagnets (pLeff octahedral v, crystal field band near 1030 nm. Characterization data are collected in the supplementary material. The complexes are stable in solutions of dichloromethane and acetonitrile. Thus, addition of bpy to Ni(RQ),(bpy) does not affect the electronic spectrum or voltammogram (see below) and upon addition of bpy to Ni(RQ), all spectral changes level off at Ni(RQ),:bpy = 1:l. Our interest in the mixed complexes lies in their possible oxidation to nickel(II1) species. The complexes indeed display a straightforward quasi-reversible cyclic voltammetric response whose current height corresponds to one-electron transfer and metal oxidation is implied by EPR results (see below). The formal potentials of the couples (eq 9) Ni"'(RQ),(N,N)+ + e- -+Ni"(RQ),(N,N) (9) correlate linearly with the pKa's of in the series Ni(MeQ),L, (L = py, mpy, apy, pz) (Table IV). A stronger donor (larger pKa) provides better stabilization for nickel(II1). In general, the formal potentials of the mixed complexes are higher than those of the tris chelates. Even though Ni(RQ),(N,N)+ exists on the cyclic voltammetric time scale, all attempts to isolate salts of the cation produced coulometrically in acetonitrile and dichloromethane solutions have failed. Irrespective of the amine ligand present, the solution coulometrically oxidized at rmm temperature (-300 K) invariably displays an EPR spectrum identical with that of mer-Ni(RQ),. The primary reaction responsible for this behavior is stated in eq 10, in which the metal oxidation equivalents are transferred from a higher to a lower potential situation via ligand redistribution. 2Ni(RQ)2(N,N)+ + Ni(RQ),(N,N) 2 mer-Ni(RQ), Ni(N,N)32+ (10) L23924
-
+
Some details will be considered with Ni(BuQ),(bpy) as the example. By performing low-temperature (263 K) coulometry at 0.9 V in d i c h l ~ r o m e t h a n eand ~ ~ immediately freezing (77 K) the partially oxidized solution, it is possible to observe an EPR spectrum that has two prominent resonances in addition to those of mer-Ni(BuQ), (Figure 3b). These resonances (gll= 2.169, g, = 2.083) are assigned to Ni(BuQ),(bpy)+. Since gll > g,, an axially compressed structure with the unpaired electron in the d.+? orbital is i n d i ~ a t e d .Nitrogen ~ hyperfine due to the oxime ligand is not observable in mer-Ni(RQ),, and the same is assumed to be true for Ni( BuQ),(bpy)+. The observed five-line hyperfine splitting (aL = 15 G) in the perpendicular region is attributed to the two bpy nitrogen atoms. The probable structure of the complex is shown by 5. We prefer the location of oximato
i
Ray and Chakravorty
7
0.751
,
/-----------
/
/ /
I
0.25t
/
/, //
/ /
1
I
0 25
I
I
0 75
0 50
100
F/mole
Figure 5. Formation of mer-Ni(BuQ), (as a fraction of the total nickel present) in the course of constant-potential electrolysis of Ni(BuQ)2(bpy) in dichloromethane solution a t 300 K: concentration of mer-Ni(BuQ), measured coulometrically (0)and spectrophotometrically (A).
can be a reason for axial compression. Quinone oxime complexes of known structures generally have M-N distances shorter than M - O distances.12 When the oxidized solution is warmed to -300 K, the spectrum of Ni(BuQ),(bpy)+ disappears and an intensified spectrum of mer-Ni(BuQ), is all that remains. From the EPR intensity, the coulometric count at 0.1 V due to reduction of Ni(BuQ), to Ni(BuQ)
